1. Field of the Invention
The present invention relates in general to a binocular indirect ophthalmoscope for stereoscopically observing an eye fundus, for example, and in particular to an apparatus of such type having a movable illuminating-light deflector and movable observation-light deflectors.
2. Description of the Prior Art
FIG. 7 shows a prior art binocular indirect ophthalmoscope of the above-described type. In this prior art apparatus, light from an illuminating light source S is projected to the fundus of an eye under test (not shown) through a projection lens P and through an illuminating-light deflector in the form of a mirror MI. In this binocular indirect ophthalmoscope, an aerial image I of the eye fundus is formed by an aspherical lens (not shown), and light from the aerial image I is split into left and right light beams by two reflecting surfaces MCl, MC2 of a center mirror MC. One of these two light beams passe via an observation-light deflector in the form of a mirror MR and via an ocular ER to the right eye of the operator, while the other light beam passes via another observation-light deflector in the form of another mirror ML and via another ocular EL to the left eye of the operator. This allows the operator to stereoscopically observe the fundus of the eye under test.
When the peripheral regions of an eye fundus should be observed by means of such a prior art indirect ophthalmoscope, it must be observed obliquely from above or from below even if the eye under test has a relatively large pupil LP (see FIG. 8A). However, there is a difficulty in that the effective illuminating light beam can be partially interrupted by the iris of an eye having a relatively large pupil LP when its fundus peripheral regions are observed obliquely from above or from below, because the relatively large pupil LP then appears elliptic, laterally elongated as shown in FIG. 8B.
To obviate such difficulty, said mirror MI may be interlocked with the other mirrors MR, ML in such a manner that the distance D between the effective observation light beams OFR, OFL on the pupil plane and the distance H to the effective illuminating light beam IF can be changed simultaneously, whereby eyes with larger pupils LP shown in FIG. 8A, as well as eyes with smaller pupils SP shown in a two-dot long and two short dashes line in FIG. 8C can be observed.
However, in such an arrangement, when the mirror MI is moved to reduce the distance to the effective illuminating light beam from H to h (H is larger than h), the other mirrors MR, ML will simultaneously move toward each other, with the result that the distance between the effective observation light beams OFR, OFL is reduced from D to d (D is larger than d), deteriorating the stereoscopic sensation as provided by such apparatus.
Further, the distance between the optical axes of the right combination of the right mirror MR and right ocular ER and of the left combination of the left mirror ML and left ocular EL must be adjustable for adaptation to the pupil-to-pupil distance IR-IL of the operator's eyes. However, the mirrors MR, ML must be movable independently from the oculars ER, EL in order that smaller-pupilled eyes can be observed. Such a requirement is not satisfied by any prior art.